Evolutionary pressures have caused the fish to develop highly modified fins to survive in its special environment.

Males also have a tough pad on the front of the pelvic girdle that may be used to help them clutch onto females during mating.

"The discovery of such a tiny and bizarre fish highlights how little we know about the diversity of Southeast Asia," says Kottelat.

"This is all the more serious because the habitat of this fish is disappearing very fast, and the fate of the species is now in doubt."

The team also found a related Paedocypris species, P. micromegethes, in Sarawak in Malaysian Borneo.

At 8.8 millimetres, the scientists say it is the second smallest freshwater vertebrate ever found.

The smallest or the shortest?

Larval fish expert, Dr Jeff Leis of the Australian Museum says P. progenetica is half a millimetre smaller than the stout infantfish (Schindleria brevipinguis), which in 2004 was announced as the word's smallest fish.

So, he says, P. progenetica is definitely the shortest fish in the world, but whether it's the smallest or not will depend on its weight.

"Weight also has to come into this," he says. "If a snake was longer than an elephant, would you say the elephant was smaller than the snake?"

Leis says that the stout infantfish, found in the coral lagoons on Australia's Great Barrier Reef, weighs less than a milligram and is lighter than the dwarf goby.

One of Kottelat's co-researchers, fish taxonomist Dr Tan Heok Hui, also of the National University of Singapore, says his team did not weigh P. progenetica.

The fish is similar in shape to the infant stoutfish, he says, and may be lighter, but length is his main criteria.

"To me the weight of a fish is never a concern," says Tan.

"The python is supposedly the longest snake in the world but it's not the heaviest because the anaconda holds the record," he says. "It depends on how you look at things."

In either case, says Leis, who doesn't think much is to be gained from such debates, the new fish is "a very interesting find".